Fabrication of three-dimensional metallic components via layer-by-layer laser cladding was first reported in 1978 by Breinan and Kear. U.S. Pat. No. 4,323,756 issued to Brown et al. in 1982, describes a method for the production of bulk rapidly solidified metallic articles of near-net shape, finding particular utility in the fabrication of certain gas turbine engine components including discs and knife-edge air seals. According to the disclosure, multiple thin layers of feedstock are deposited using an energy beam to fuse each layer onto a substrate. The energy source employed may be a laser or an electron beam. The feedstock employed in the practice of the invention may be either a wire or powder material, and this feedstock is applied to the substrate in such a fashion that it passes through the laser beam and fuses to the melted portion of the substrate.
These methods of direct metal deposition may make use of multi-axis, commercially available CNC machines for the fabrication of three-dimensional components. U.S. Pat. No. 5,837,960 relates to a method and apparatus for forming articles from materials in particulate form. The materials are melted by a laser beam and deposited at points along a tool path to form an article of the desired shape and dimensions. Preferably the tool path and other parameters of the deposition process are established using computer-aided design and manufacturing techniques. A controller comprised of a digital computer directs movement of a deposition zone along the tool path and provides control signals to adjust apparatus functions, such as the speed at which a deposition head which delivers the laser beam and powder to the deposition zone moves along the tool path.
Most existing techniques, however, are based on open-loop processes requiring either a considerable amount of final machining to achieve a part of close dimensional tolerances. Continuous corrective measures during the manufacturing process are necessary to fabricate net shape functional parts with close tolerances and acceptable residual stress. A closed loop system which achieves these goals is described in U.S. Pat. No. 6,122,564. This patent discloses a laser-aided, computer-controlled direct-metal deposition, or DMD, system wherein successive layers of material are applied to a substrate so as to fabricate an object or to provide a cladding layer.
In contrast to previous methodologies, this DMD system is equipped with feedback monitoring to control the dimensions and overall geometry of the fabricated article in accordance with a computer-aided design (CAD) description. The deposition tool path is generated by a computer-aided manufacturing (CAM) system for CNC machining, with post-processing software for deposition, instead of software for removal as in conventional CNC machining. Such feedback controlled systems may totally eliminate intermediate machining and reduce final machining considerably.
U.S. Pat. No. 6,518,541 discloses a direct metal deposition system employing a laser having an on/off duty cycle and means for controlling the process parameters through feedback to maintain the duty cycle within a desired range. U.S. Pat. No. 6,459,951 relates to a DMD system employing feedback control of the system parameters to maintain a constant well pool as the deposition progresses on a particular layer.
These feedback control systems improve the precision of each layer of the multi-layer fabrication process but do not address the problem of layer-to-layer variations which result from heating of the substrate as the DMD process progresses.